Method and arrangement for adaptive dispersion compensation

ABSTRACT

A received optical signal is coherently demodulated and converted into electrical complex samples, which are dispersion compensated in a compensation filter. A control circuit calculates comparison values from corrected samples and an estimated error value. A plurality of compensation functions is applied according to a predetermined dispersion range and after a second iteration, the compensation filter is set to an optimum compensation function.

FIELD OF THE INVENTION

The invention refers to a method and an arrangement for adaptivedispersion compensation. The invention may be used for all kinds ofoptical transmission formats including polarisation multiplextransmission.

BACKGROUND OF THE INVENTION

In order to meet the growing demand for internet bandwidth with trafficgrowth rates around 40-50% per year, telecommunication componentproviders face the task of increasing the spectral efficiency of fiberutilization. After 10 Gbit/s systems (G-Giga) became successful in the1990's, solutions for 40 Gbit/s became available in the last years.Standardization and research are now focused on the development of 100Gbit/s systems with coherent polarization multiplexed (CP) QPSK(Quadrature Phase Shift Keying) being the most likely modulation formatfor next generation systems. Since polarization multiplexing utilizesboth light polarizations, it is possible to send the signal at a rate of˜25-28 G symbols per second, thus fitting nicely into the standard 50GHz grid for DWDM (Dense Wavelength Diversity Multiplex) opticalsystems.

E.g. Seb J. Savory, “Digital filters for coherent optical receivers”,Optics Express 16, No. 2, pp. 804-817, 9. Jan. 2008 describes theprinciple of a polarisation diversity multiplex (polmux) transmissionsystem with dispersion compensation.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide methods and arrangements foradaptive dispersion compensation as well as equalization of arbitraryother linear distortions (e.g. filters) that can be described by a setof transfer functions.

The object is achieved by the features recited in method claims 1, 2,and in arrangement claims 12 and 13. A coherent polarisation diversitymultiplex receiver is described in claim 18.

According to the present invention there is provided a method foradaptive dispersion compensation, comprising the steps of

-   -   demodulating a received optical signal and converting it into        digital samples,    -   feeding the samples to a compensation filter,    -   setting the compensation filter to a compensation function        according to a dispersion value,    -   outputting corrected samples,    -   calculating comparison values from the corrected samples,    -   calculating an error value from the corrected samples and        storing an estimated minimum error value,    -   applying further compensation functions according to further        dispersion values altered by variations within a predetermined        dispersion range, and    -   setting an optimal filter compensation function according to the        minimum error value.

Because the adaptation length of a time domain equalizer increasesexponentially with the signal spread this method is suitable formoderate dispersion values and short transmission links, e.g. up to 200km.

There is also provided a method for adaptive dispersion compensationcomprising the steps of

-   -   demodulating a received optical signal and converting it into        digital samples,    -   feeding N samples to a FFT unit,    -   converting the N samples into a spectral function,    -   setting a compensation unit to a compensation function according        to a dispersion value,    -   converting the corrected spectral function into corrected        samples,    -   calculating comparison values from the corrected samples,    -   calculating an error value from the corrected samples and        storing an estimated minimum error value,    -   applying further compensation function according to further        dispersion values altered by a variation within a predetermined        dispersion range, and    -   setting an optimal filter compensation function according to the        minimum error value.

This method is suited for transmission links, e.g., from 200 km andbeyond.

Further advantageous features are described in the pending claims.

Coherent reception makes it possible to compensate for large values ofchromatic dispersion using digital signal processing, thus allowing toeliminate optical dispersion compensating fibers, and to reduce thenumber of amplifiers, saving costs in the system. Dispersion is a lineareffect that can be described analytically by an all-pass transferfunction. An approximation omitting higher order terms is given by

${G\left( {z,\omega} \right)} = {\exp\left( {{- j}\; D\;\frac{\lambda^{2}}{2\pi\; c}\frac{\omega^{2}}{2}z} \right)}$where D is the dispersion parameter of the fiber in [ps/nm/km], λ is thereference wavelength, ω is the angular frequency offset from thereference frequency, c is the speed of light and z is the transmissiondistance in km. If the total dispersion given by D·z [ps/nm] is known,the filter can be set to the inverse dispersion value and any arbitraryvalue of dispersion can be compensated without penalty, if the signalwas at least two-fold oversampled. Thus, the estimation of thedispersion value D·z in the receiver without any transmitted trainingsymbols is the key component to fully exploit capabilities of nextgeneration coherent optical systems, a problem that is solved by theinvention. In addition, the invention provides the according filtercompensation function at the same step.

The estimation algorithm can be implemented either for a time-domainequalizer or for a frequency-domain equalizer, and delivers identicalresults. Here, shorter links about 70-200 km might be a candidate formoderate-length TDEs (time domain equalizers), whereas longertransmission links starting at 200/300 km should require FDE (frequencydomain equalizers) in order to keep complexity low. The advantages ofthe proposed solution are as follows:

-   -   Estimation is blind and does not require signal overhead,        training symbols or a feedback channel to the transmitter,    -   No additional correlation filters are necessary,    -   The estimation precision is very high,    -   The predefined filter functions immediately compensates for the        estimated chromatic dispersion thus making it possible to        implement a time domain (TD) finite impulse response (FIR)        filter with a low number of taps, reducing complexity,    -   The adaptation length is in the range of a few microseconds        regardless of the actual value of dispersion,    -   The dispersion compensator cannot misconverge, in contrary to        straight-forward adaptation algorithms,    -   Computational complexity is very low requiring one        multiplication per symbol used for estimation (additions and        subtractions can be neglected for complexity computations), and    -   Once the correction filter or the correction unit is set, no        continuous update is required. Tapping the signal from time to        time, the value of chromatic dispersion can be monitored in an        off-line mode.

The invention is applicable for non-coherent and coherent demodulation.Coherent reception enables also the separation of the polarisationcomponent signals of a polarisation diversity signal without apolarisation control. Therefore the invention is especially suited forpolarisation multiplex diversity systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention including a presently preferred embodiment aredescribed below with reference to accompanying drawings, where

FIG. 1 is a schematic block diagram of receiver,

FIG. 2 is a schematic block diagram of a time domain dispersioncompensation unit,

FIG. 3 shows a simplified block diagram of a frequency domain dispersioncompensation unit,

FIG. 4 shows diagrams for a first and second iteration,

FIG. 5 shows a performance diagram of the frequency domain dispersioncompensation unit,

FIG. 6 is a schematic block diagram of a coherent receiver withpolarisation demultiplexing, and

FIG. 7 shows a schematic block diagram of a dispersion compensation unitand butterfly stages.

DETAILED DESCRIPTION OF THE INVENTION

Different embodiments of the invention suited for different kinds ofsystems will now be described.

FIG. 1 shows a schematic bloc diagram of an optical receiver 5,6,10. Ina demodulation and conversion unit 5 a received optical signal S_(IN) isconverted into digital samples p(n). The digital samples are digitallycorrected in a dispersion compensation unit 6 and outputted as correctedsamples q(n), which are fed to a decision unit 10 which outputs a datasignal D_(OUT).

The invention can also be applied for coherent demodulation. Then aconstant wave signal CW is generated by a local oscillator 2 and fed toan adequate demodulation and conversion unit 5. In this case p(n) andq(n) become complex values.

FIG. 2 shows a first embodiment of the dispersion compensation unit(equaliser) 61/62 for time domain dispersion compensation. The inputsamples p(n) may be real or complex samples. Or, if polarisationdiversity multiplex modulation is applied, orthogonal samplespx(n)/py(n). The samples are fed to a digital M stage FIR compensationfilter 11. The dispersion is compensated by an inverse filter function.The filter coefficients are determined and set by applying a filtercompensation function. An optimal filter compensation function isdetermined in a control circuit including a control unit 14, a referencevalues R1, R2 calculation unit 12, and a comparator 13. A delay element(D) 16 is inserted to indicate correct timing. A plurality of filtercompensation functions T(M) corresponding to a plurality of dispersionvalues is stored in a function storage 15 (or calculated). A controlsection 141 of the control unit 14 reads these filter compensationfunctions from the function storage 15 (look up table), and a minimumestimated error value ε_(MIN) is calculated by adder 13 by comparing thecorrected samples q(x) and the reference values R1, R2 and stored in ancalculation section 142, which is here also a part of the control unit14. The filter compensation function generating the estimated minimumerror value ε_(MIN) is selected for compensation. The corrected samplesq(n) or qx(n) and qy(n) are output signals of the compensation filter11.

Instead of allowing the filter coefficients to adapt freely, thuscausing the high risk of a misconverged equalizer, all possiblecompensation functions for corresponding chromatic dispersion values arepreloaded into the function store as a lookup table. The compensationfunctions correspond to chromatic dispersion values ranging from aminimum to a possible maximum value in the system in certain steps.Initially, each of the compensating functions from the lookup table isapplied and the best matching filter function is chosen with the aid ofan error criterion defined below.

FIG. 3 shows an embodiment of a dispersion equaliser 61F/62F for thefrequency domain suited for higher dispersion values. For higherdispersion values this arrangement has a complexity advantage over atime domain dispersion compensator.

The real or complex input samples p(n) or px(n)/py(n) are stored in aFast Fourier Transformation (FFT) unit 16, and N samples together areconverted into a spectrum Ω(N). The N coefficients representing thisspectrum are multiplied in a compensation unit 17 by a spectralcompensation function C(N). The compensation unit 17 corresponds to thecompensation filter 11 in the time domain equaliser. The compensatedspectrum ΩC(N)=Ω(N)·C(N) is then converted by an Inverse Fast FourierTransformation (IFFT) unit into corrected real or complex time domainsamples q(n) or qx(n)/qy(n).

After the reception of e.g. N/2 new samples p(n), px(n)/py(n) theconversion process is repeated but with another compensation functionC(N) corresponding to a different dispersion. Again, the control section141 of the control unit 14 reads these spectral compensation functionsfrom the function storage 15 and all the spectral compensation functionsare tested within a predefined dispersion range. The spectralcompensation function generating the estimated minimum error valueε_(MIN) is selected for compensation.

The same algorithm is applied for TDE (time domain equalisation) and FDE(frequency domain equalisation). To calculate estimated errors “ε” atleast a reference value has to be derived from corrected samples q(n)(px(n), py(n) respectively), which are output signals of thecompensation filter 11. The error criterion is derived for the case oftwo-fold oversampling of the signal according to an article: DominiqueN. Godard, “Self-Recovering Equalization and Carrier Tracking inTwo-Dimensional Data Communication Systems”, IEEE Transactions onCommunications, vol. COM-28, No. 11, pp. 1867-1875, November 1980.

The proposed invention however does not utilize the error signal toadapt the coefficients of the compensation function/filter, whichimproves the convergence properties and at the same time stronglyreduces the implementation complexity, but instead tries to minimize thetotal error power in an error criterion given by

${{ɛ({CD})} = {\sum\limits_{n = 1}^{N}\left( {{{{{q\left\lbrack {{2n} - 1} \right\rbrack}}^{2} - R_{1}}} + {{{{q\left\lbrack {2n} \right\rbrack}}^{2} - R_{2}}}} \right)}},{{\forall{W_{CD}(\Omega)}} = \left\lbrack {{CD}_{m\; i\; n}:{\delta\;{{CD}:{CD}_{{ma}\; x}}}} \right\rbrack},$N—number of samples; δCD—dispersion variation, W_(CD)(Ω)—set of possiblevalues for the free parameter (chromatic dispersion CD); referencevalues R₁ and R₂ are estimated average values from the power of odd andeven corrected samples q(n), unlike conventional adaptation algorithms,thus serving as a basic timing recovery. The values of R₁ and R₂ followthe changing timing phase, making this approach work in the first place.In order to simplify the evaluation of the above equation in high speedimplementations, the values R₁ and R₂ can also be calculated for exampleaccording to the following formula:

power ratio R₁ R₂ $\frac{q_{2,{av}}}{q_{1,{av}}} > \xi$ R_(a) R_(c)$\frac{1}{\xi} \leq \frac{q_{2,{av}}}{q_{1,{av}}} \leq \xi$ R_(b) R_(b)$\frac{q_{2,{av}}}{q_{1,{av}}} < \frac{1}{\xi}$ R_(c) R_(a)where q_(1,av) and q_(2,av) are the time averages of the power of theodd and even samples of the corrected signal q(n), and ξ, R_(a), R_(b),and R_(c) are constants to be optimized for the specific modulationformat, pulse shape etc. W_(CD)(Omega) is the set of values of aparameter (e.g. dispersion CD) that are to be examined as possiblesolutions. The equation is valid for two-fold oversampling. The chosensampling rate is slightly higher. Of course, more than two-foldoversampling is also possible but difficult because the data rate ishigh.

A first iteration starts at a chromatic dispersion CD_(min), e.g.=0ps/nm. The applied dispersion to the compensation filter 11,respectively the compensation unit 18, is increased in steps of δCD(e.g. δCD=200 ps/nm) over all dispersion values of a predetermineddispersion range up to CD_(max). A control section 141 of the unit 14reads these correction functions from the function storage 15 (look uptable). The actual minimum error value ε_(MIN) is stored in thecalculation section 142 of the control unit.

The estimated errors are shown in a diagram FIG. 4 (a). The minimumerror value ε_(MIN) can easily be recognized.

The error values ε_(MIN) can be used immediately to obtain the optimumsolution (corresponding to minimum ε_(MIN)) or the error values can beaveraged over several consecutive blocks to increase estimationaccuracy.

After determining the best inverse dispersion value of the compensationfunction a second iteration or even further iterations can be repeatedwith higher resolution in the minimum error range.

The second iteration with a 5-20 times higher resolution of δCD (e.g.δCD=20 ps/nm) is performed in the minimum error range, which can berestricted to ±3×δCD (e.g. ±3×δCD=±600 ps/nm) symmetrically around theoptimum found in the previous iteration. The error values and acalculated sliding average error value according to the second iterationare shown in FIG. 4( b). A calculated minimum ε_(MINC) of the slidingaverage error values gives the exact dispersion correction value.

The compensation filter 11 respectively the compensation unit 18 is thenset to the optimal compensation function=inverse dispersion value with alowest error for optimum compensation.

FIG. 5 shows the required OSNR (optical signal to noise ratio) as afunction of the chromatic dispersion for a bit error rate of 1 10⁻³. Thediagram shows the performance of a system for 112 Gbit/s and severalFFT-sizes with N/2 overlap.

The estimation quality is highly precise even in presence of otherchannel distortions and strong noise and can be further improved for alarger number of symbols used for estimation.

Of course, the invention is especially suited for polmux systems.Therefore, the invention will now be described in detail as a part of apolarisation division multiplex (polmux) transmission system. Thissystem transmits two optical signals with the same carrier wavelengthbut orthogonal polarisation in a single transmission channel. Thereceived optical polarisation division multiplex signal may be splitinto the transmitted two optical signals and then demodulated. But thismethod requires a polarisation control of the received polmux(polarisation division multiplex) signal.

FIG. 6 shows a schematic block diagram of a today's polarisationdivision multiplex receiver. A received polmux signal S_(H), S_(V)(H—horizontal, V—vertical) carrying the information of a first datasignal H and a second data signal V is split by a polarisation beamsplitter 1 into two orthogonal component signals Sx with x-polarisationand Sy with orthogonal y-polarisation. A local oscillator 2 generates aconstant wave signal which is also split into two orthogonally polarizedconstant wave signals and fed together with the orthogonal componentsignals Sx and Sy to two 90°-hybrids 3 and 4, where each orthogonalcomponent signal Sx and Sy is converted into two orthogonal x-componentsx_(I), x_(Q) and into two y-components y_(I), y_(Q) respectively (alsoreferred to as in-phase component I, quadrature component Q; or real andimaginary part). These x-components and y-components are separatelyconverted by demodulation and converter units 51-54 into digital complexsamples px(n)=X_(I)(n), X_(Q)(n) (=X_(I)(n)+j X_(Q)(n)) andpy(n)=Y_(I)(n), Y_(Q)(n) in the electrical domain. The complex samplesX_(I)(n), X_(Q)(n) and Y_(I)(n), Y_(Q)(n) still carry the information ofthe optical component signals Sx and Sy (which are usually not thetransmitted signals but contain parts of the two optical signals S_(H)and S_(V)). Two dispersion compensation units 61, 62 (respectively61F/62F) correct separately the orthogonal samples X_(I)(n), X_(Q)(n) ofthe x-component signal and the orthogonal samples Y_(I)(n), Y_(Q)(n) ofthe y-component signal and output corrected complex samplesqx(n)=X_(IC)(n), X_(QC)(n) and qy(t)=Y_(IC)(n), Y_(QC)(n) respectively.Ignoring the timing recovery 7, known to those skilled in the art andnot a part of the invention, the corrected complex samples qx(n) andqy(n) are fed to a FIR (finite impulse response) butterfly filter 8 (adigital processor unit), which reconstructs the two transmitted signalsS_(H), S_(V) in a sample format H_(I)(n), H_(Q)(n) and V_(I)(n),V_(Q)(n). A following carrier recovery unit 9 corrects frequency andphase difference of the local oscillator 2. And a symbol decision unit10 compares the reconstructed samples (symbols) with threshold valuesand outputs data signals H_(OUT) and V_(OUT) (e.g. if a symbolcorresponds to two bits) H_(OUT)=H_(OUTI), H_(OUTQ); V_(OUT)=V_(OUTI),V_(OUTQ); I, Q—components are not shown).

As mentioned before, the x-polarisation signal and y-polarisation signalof a polmux signal are separately compensated in these embodiments.Common compensation is of course also possible. A low cost solution mayuse only one compensation circuit and use the same compensation functionfor both polarisation signals.

FIG. 7 shows a more detailed block diagram of the chromatic dispersioncompensation (CDC) units 61/61F, 62/62F and the butterfly filter 8connected in series (clock recovery block not shown). The butterflyfilter includes four FIR filters and two adders A1, A2.

The complex samples px(n) of the component signal Sx and py(n) of thecomponent signal Sy are dispersion separately corrected and thecorrected complex samples qx(n) and qy(n) are fed to the butterflyfilter 8. The butterfly filter 8 reconstructs the received polmuxsignals S_(H) and S_(V) according to adaptive transfer functionsh_(xx)−h_(yy) in the form of corrected complex samples h(n)=H_(I)(n),H_(Q)(n) and v(n)=V_(I)(n), V_(Q)(n).

It is also possible to derive the number of taps required for thefollowing TD FIR butterfly filter from the standard deviation of theestimation error.

If a conventional polmux receiver comprising a polarisation control andperforming non-coherent demodulation is used both compensation units 61,62 are designed for processing real sample values only.

As mentioned before, the x-polarisation signal and y-polarisation signalof a polmux signal are separately compensated in these embodiments. Alow cost solution may use only one control circuit and the samecompensation function for both polarisation signals.

The present invention is not limited to the details of the abovedescribed principles. The scope of the invention is defined by theappended claims and all changes and modifications as fall within theequivalents of the scope of the claims are therefore to be embraced bythe invention.

REFERENCE SIGNS

-   1 polarisation beam splitter-   2 local oscillator-   3 first 90° hybrid-   4 second 90° hybrid-   5 demodulation and converter unit-   6 dispersion compensation unit-   61 first dispersion compensation unit-   62 second dispersion compensation unit-   7 clock recovery unit-   8 FIR butterfly filter-   9 carrier recovery unit-   10 symbol decision unit-   11 FIR compensation filter-   12 reference value calculation unit-   13 comparison unit-   14 control unit-   141 control section-   142 error storage-   15 function storage-   16 FFT unit-   17 correction unit-   18 IFFT unit-   19 delay-   S_(IN) received signal-   CW constant wave signal-   S_(IH), S_(IV) received polmux signal-   Sx, Sy component signals-   X_(I), X_(Q) orthogonal x-components-   y_(I), y_(Q) orthogonal y-components-   p(n) digital samples-   X_(I)(n), X_(Q)(n) complex x-samples-   Y_(I)(n), Y_(Q)(n) complex y-samples-   X_(IC)(n), X_(QC)(n) corrected x-samples-   Y_(IC)(n), Y_(QC)(n) corrected y-samples-   H_(OUT), V_(OUT) output signal values-   p(n) digital samples-   px(n) complex x-samples-   py(n) complex y-samples-   p(n) complex samples-   h transfer function-   q(n) corrected samples-   qx(n) corrected samples of px(n)-   qy(n) corrected samples of py(n)-   hx(n) reconstructed H signal-   hy(n) reconstructed V signal-   H_(I)(n), H_(Q)(n) samples of the H signal-   V_(I)(n), V_(Q)(n) samples of the V signal-   T(M) filter compensation function-   C(N) spectral compensation function-   ε estimated error value-   ε_(MIN) estimated minimum error value-   ε_(MINC) calculated minimum error value-   R₁, R₁ reference values

The invention claimed is:
 1. A method for adaptive dispersioncompensation, which comprises the steps of: demodulating a receivedoptical signal and converting the received optical signal into digitalsamples; feeding the digital samples to a compensation filter; settingthe compensation filter to a filter compensation function according to adispersion value; outputting corrected samples from the compensationfilter; calculating comparison values from the corrected samples;calculating an estimated error value from the corrected samples andstoring an estimated minimum error value; applying further compensationfunctions according to further dispersion values altered by dispersionvariations within a predetermined dispersion range; and setting thecompensation filter to an optimal filter compensation function assignedto the estimated minimum error value.
 2. The method according to claim1, which further comprises performing at least a second iterationprocess with higher resolution applying smaller dispersion variations ina minimum error value range of a preceding iteration process.
 3. Themethod according to claim 2, which further comprises calculating slidingaverage error values and setting the compensation filter or acompensation unit according to an optimal dispersion value associated toa calculated minimum error value.
 4. The method according to claim 1,which further comprises applying twofold oversampling and deriving afirst comparison value from an average power of odd samples and a secondcomparison value from an average power of even samples.
 5. The methodaccording to claim 4, which further comprises calculating estimatederror values according to${{ɛ({CD})} = {\sum\limits_{n = 1}^{N}\left( {{{{{q\left\lbrack {{2n} - 1} \right\rbrack}}^{2} - R_{1}}} + {{{{q\left\lbrack {2n} \right\rbrack}}^{2} - R_{2}}}} \right)}},{{\forall{W_{CD}(\Omega)}} = \left\lbrack {{CD}_{m\; i\; n}:{\delta\;{{CD}:{CD}_{{ma}\; x}}}} \right\rbrack},$with q(n) being the corrected samples, R₁ being the average power of theodd samples, R₂ being the average power of the even samples, δCD beingthe dispersion variation, and W_(CD)(Ω) being a set of possible valuesfor chromatic dispersion CD.
 6. The method according to claim 1, whichfurther comprises the step of reading the compensation functions from afunction storage unit.
 7. The method according to claim 1, which furthercomprises the step of calculating the compensation functions accordingto different dispersion values in a predetermined range.
 8. The methodaccording to claim 1, which further comprises the steps of coherentlydemodulating and converting the received optical signal into complexsamples, which are corrected into corrected complex samples.
 9. Themethod according to claim 1, which further comprises the steps of:receiving an optical polarization diversity multiplex signal andconverting the optical polarization diversity multiplex signal intoorthogonal x-components and y-components; sampling the orthogonalx-components and y-components and converting them into digital complexsamples; and correcting separately the digital complex samples.
 10. Amethod for adaptive dispersion compensation, which comprises the stepsof: demodulating a received optical signal and converting the receivedoptical signal into digital samples; feeding the digital samples to aFast Fourier Transformation unit; converting the digital samples into aspectral function; setting a compensation unit to a spectralcompensation function according to a dispersion value; correcting thespectral function via the compensation unit and outputting a correctedspectral function; converting the corrected spectral function intocorrected samples, via an Inverse Fast Fourier Transformation;calculating comparison values from the corrected samples; calculating anestimated error value from the corrected samples and storing anestimated minimum error value; applying a further spectral compensationfunction according to further dispersion values altered by dispersionvariations within a predetermined dispersion range; and setting thecompensation unit to an optimal spectral compensation function assignedto the estimated minimum error value.
 11. The method according to claim10, which further comprises performing at least a second iterationprocess with higher resolution applying smaller dispersion variationswith a minimum error value range of a preceding iteration process. 12.The method according to claim 11, which further comprises calculatingsliding average error values and setting the compensation filter or acompensation unit according to an optimal dispersion value associated toa calculated minimum error value.
 13. The method according to claim 10,which further comprises storing 0.25N-0.75N new samples and convertingthe new samples and 0.75N-0.25N former samples into a next spectralfunction, where N=number of stored samples.
 14. The method according toclaim 10, which further comprises applying twofold oversampling andderiving a first comparison value from an average power of odd samplesand a second comparison value from an average power of even samples. 15.The Method according to claim 14, which further comprises calculatingestimated error values (c) according to${{ɛ({CD})} = {\sum\limits_{n = 1}^{N}\left( {{{{{q\left\lbrack {{2n} - 1} \right\rbrack}}^{2} - R_{1}}} + {{{{q\left\lbrack {2n} \right\rbrack}}^{2} - R_{2}}}} \right)}},{{\forall{W_{CD}(\Omega)}} = \left\lbrack {{CD}_{m\; i\; n}:{\delta\;{{CD}:{CD}_{{ma}\; x}}}} \right\rbrack},$with q(n) being the corrected samples, R₁ being the average power of theodd samples, R₂ being the average power of the even samples, δCD beingthe dispersion variation, and W_(CD)(Ω) being a set of possible valuesfor chromatic dispersion CD.
 16. The method according to claim 10, whichfurther comprises the step of reading the compensation functions from afunction storage unit.
 17. The method according to claim 10, whichfurther comprises the step of calculating the compensation functionsaccording to different dispersion values in a predetermined range. 18.The method according to claim 10, which further comprises the steps ofcoherently demodulating and converting the received optical signal intocomplex samples, which are corrected into corrected complex samples. 19.The method according to claim 10, which further comprises the steps of:receiving an optical polarization diversity multiplex signal andconverting the optical polarization diversity multiplex signal intoorthogonal x-components and y-components; sampling the orthogonalx-components and y-components and converting them into digital complexsamples; and correcting separately the digital complex samples.
 20. Aconfiguration for dispersion compensation, comprising: a receiver havinga demodulation and converter unit outputting digital samples of areceived optical signal, and at least one dispersion compensator; saidat least one dispersion compensator containing: a compensation filterreceiving the digital samples and outputting corrected samples; areference value calculation unit receiving the corrected samples andcalculating at least two comparison values; a comparison unit comparingthe corrected samples with the comparison values and outputting anestimated error value; and a control unit setting said compensationfilter to further filter compensation functions according to furtherdispersion values, to calculate estimated error values, and to store anestimated minimum error value, said control unit selecting an optimalfilter compensation functions assigned to the estimated minimum errorvalue.
 21. The configuration according to claim 20, further comprising afunction store storing a plurality of compensation functions.
 22. Theconfiguration according to claim 20, wherein said control unit has acontrol circuit calculating estimated error values according${{ɛ({CD})} = {\sum\limits_{n = 1}^{N}\left( {{{{{q\left\lbrack {{2n} - 1} \right\rbrack}}^{2} - R_{1}}} + {{{{q\left\lbrack {2n} \right\rbrack}}^{2} - R_{2}}}} \right)}},{{\forall{W_{CD}(\Omega)}} = \left\lbrack {{CD}_{m\; i\; n}:{\delta\;{{CD}:{CD}_{{ma}\; x}}}} \right\rbrack},$with q(n) is the corrected samples, R₁ is an average power of oddsamples, R₂ is an average power of even samples, and W_(CD)(Ω) is a setof possible values for chromatic dispersion CD.
 23. The configurationaccording to claim 20, wherein said control unit performs a firstiteration over a predetermined dispersion range and at least a seconditeration over a range including the estimated minimum error value. 24.The configuration according to claim 20, wherein said control unitcalculates a sliding average error and sets the compensation functionaccording to the estimated minimum error value.
 25. The configurationaccording to claim 20, wherein said dispersion compensation unit isconfigured for processing and correcting complex digital samples.
 26. Aconfiguration for dispersion compensation, comprising: a receiver havinga demodulation and converter unit outputting digital samples, and atleast one dispersion compensator unit; said at least one dispersioncompensator unit containing: a Fast Fourier Transformation unitconverting the digital samples into a spectral function; a correctionunit receiving the spectral function and outputting a corrected spectralfunction; an Inverse Fast Fourier Transformation unit converting thecorrected spectral function into corrected samples; a reference valuecalculation unit receiving the corrected samples and calculatingcomparison values; a comparison unit comparing compensated samples withthe comparison values and outputting an estimated error value; and acontrol unit setting a compensation unit to further spectralcompensation functions according to further dispersion values, tocalculate estimated error values, and to store an estimated minimumerror value, said control unit selecting an optimal spectralcompensation function assign to the minimum error value.
 27. Theconfiguration according to claim 26, further comprising a function storestoring a plurality of compensation functions.
 28. The configurationaccording to claim 26, wherein said control unit has a control circuitcalculating the estimated error values according${{ɛ({CD})} = {\sum\limits_{n = 1}^{N}\left( {{{{{q\left\lbrack {{2n} - 1} \right\rbrack}}^{2} - R_{1}}} + {{{{q\left\lbrack {2n} \right\rbrack}}^{2} - R_{2}}}} \right)}},{{\forall{W_{CD}(\Omega)}} = \left\lbrack {{CD}_{m\; i\; n}:{\delta\;{{CD}:{CD}_{{ma}\; x}}}} \right\rbrack},$with q(n) is the corrected samples, R₁ is an average power of oddsamples, R₂ is an average power of even samples, and W_(CD)(Ω) is a setof possible values for chromatic dispersion CD.
 29. The configurationaccording to claim 26, wherein said control unit performs a firstiteration over a predetermined dispersion range and at least a seconditeration over a range including the estimated minimum error value. 30.The configuration according to claim 26, wherein said control unitcalculates a sliding average error and sets the compensation functionaccording to an estimated minimum error value.
 31. The configurationaccording to claim 26, wherein said dispersion compensation unit isconfigured for processing and correcting complex digital samples.